Commit message: i2c: fix stack buffer overflow vulnerability in i2c md command

When running "i2c md 0 0 80000100", the function do_i2c_md parses the
length into an unsigned int variable named length. The value is then
moved to a signed variable:

    int nbytes = length;
    #define DISP_LINE_LEN 16
    int linebytes = (nbytes > DISP_LINE_LEN) ? DISP_LINE_LEN : nbytes;
    ret = dm_i2c_read(dev, addr, linebuf, linebytes);

On systems where integers are 32 bits wide, 0x80000100 is a negative
value to "nbytes > DISP_LINE_LEN" is false and linebytes gets assigned
0x80000100 instead of 16.

The consequence is that the function which reads from the i2c device
(dm_i2c_read or i2c_read) is called with a 16-byte stack buffer to fill
but with a size parameter which is too large. In some cases, this could
trigger a crash. But with some i2c drivers, such as drivers/i2c/nx_i2c.c
(used with "nexell,s5pxx18-i2c" bus), the size is actually truncated to
a 16-bit integer. This is because function i2c_transfer expects an
unsigned short length. In such a case, an attacker who can control the
response of an i2c device can overwrite the return address of a function
and execute arbitrary code through Return-Oriented Programming.

Fix this issue by using unsigned integers types in do_i2c_md. While at
it, make also alen unsigned, as signed sizes can cause vulnerabilities
when people forgot to check that they can be negative.

Signed-off-by: Nicolas Iooss <nicolas.iooss+uboot@ledger.fr>
Reviewed-by: Heiko Schocher <hs@denx.de>
CVE: CVE-2022-34835
CWE: CWE-787
Vulnerability description: In Das U-Boot through 2022.07-rc5, an integer signedness error and resultant stack-based buffer overflow in the "i2c md" command enables the corruption of the return address pointer of the do_i2c_md function.

Project: u-boot
File: i2c.c

static int do_i2c_md(struct cmd_tbl *cmdtp, int flag, int argc,
		     char *const argv[])
{
	uint	chip;
	uint	addr, length;
	int alen;
	int	j, nbytes, linebytes;
	int ret;
#if CONFIG_IS_ENABLED(DM_I2C)
	struct udevice *dev;
#endif

	/* We use the last specified parameters, unless new ones are
	 * entered.
	 */
	chip   = i2c_dp_last_chip;
	addr   = i2c_dp_last_addr;
	alen   = i2c_dp_last_alen;
	length = i2c_dp_last_length;

	if (argc < 3)
		return CMD_RET_USAGE;

	if ((flag & CMD_FLAG_REPEAT) == 0) {
		/*
		 * New command specified.
		 */

		/*
		 * I2C chip address
		 */
		chip = hextoul(argv[1], NULL);

		/*
		 * I2C data address within the chip.  This can be 1 or
		 * 2 bytes long.  Some day it might be 3 bytes long :-).
		 */
		addr = hextoul(argv[2], NULL);
		alen = get_alen(argv[2], DEFAULT_ADDR_LEN);
		if (alen > 3)
			return CMD_RET_USAGE;

		/*
		 * If another parameter, it is the length to display.
		 * Length is the number of objects, not number of bytes.
		 */
		if (argc > 3)
			length = hextoul(argv[3], NULL);
	}

#if CONFIG_IS_ENABLED(DM_I2C)
	ret = i2c_get_cur_bus_chip(chip, &dev);
	if (!ret && alen != -1)
		ret = i2c_set_chip_offset_len(dev, alen);
	if (ret)
		return i2c_report_err(ret, I2C_ERR_READ);
#endif

	/*
	 * Print the lines.
	 *
	 * We buffer all read data, so we can make sure data is read only
	 * once.
	 */
	nbytes = length;
	do {
		unsigned char	linebuf[DISP_LINE_LEN];
		unsigned char	*cp;

		linebytes = (nbytes > DISP_LINE_LEN) ? DISP_LINE_LEN : nbytes;

#if CONFIG_IS_ENABLED(DM_I2C)
		ret = dm_i2c_read(dev, addr, linebuf, linebytes);
#else
		ret = i2c_read(chip, addr, alen, linebuf, linebytes);
#endif
		if (ret)
			return i2c_report_err(ret, I2C_ERR_READ);
		else {
			printf("%04x:", addr);
			cp = linebuf;
			for (j=0; j<linebytes; j++) {
				printf(" %02x", *cp++);
				addr++;
			}
			puts ("    ");
			cp = linebuf;
			for (j=0; j<linebytes; j++) {
				if ((*cp < 0x20) || (*cp > 0x7e))
					puts (".");
				else
					printf("%c", *cp);
				cp++;
			}
			putc ('\n');
		}
		nbytes -= linebytes;
	} while (nbytes > 0);

	i2c_dp_last_chip   = chip;
	i2c_dp_last_addr   = addr;
	i2c_dp_last_alen   = alen;
	i2c_dp_last_length = length;

	return 0;
}